Thermoplastic film, and in particular polyolefin materials, have been used for some time in connection with packaging of various articles including food products which require protection from the environment, an attractive appearance, and resistance to abuse during the storage and distribution cycle. Suitable optical properties are also desirable in order to provide for inspection of the packaged product after packaging, in the distribution chain, and ultimately at point of sale. Optical properties such as high gloss, high clarity, and low haze characteristics contribute to an aesthetically attractive packaging material and packaged product to enhance the consumer appeal of the product. Various polymeric materials have been used to provide lower gas permeability in order to reduce the transmission of oxygen through the packaging film and thereby retard the spoilage and extend the shelf life of products such as food items which are sensitive to oxygen.
In some packaging applications, such as the packaging of roasted chicken and pizza, it is desirable to provide a packaging film with a combination of high abuse resistance and good oxygen barrier properties. This combination of properties provides adequate physical protection for the packaged item during storage and distribution, as well as the necessary shelf life.
Some cheese products are produced in such a way that the final cheese product emits a significant amount of carbon dioxide over time . In such cases, it is often desirable to provide a packaging material which is characterized by a relatively low oxygen transmission rate (i.e. good oxygen barrier), and a relatively high carbon dioxide transmission rate. A preferred O.sub.2 transmission rate is no more than about 500 cc/square meter (ASTM D 3985), more preferably no more than about 250 cc/square meter. A preferred CO.sub.2 transmission rate is at least about 750 cc/square meter, more preferably at least about 1000 cc/square meter, most preferably at least about 1200 cc/square meter. The ratio of CO.sub.2 transmission rate to O.sub.2 transmission rate is preferably greater than about 1:1, more preferably at least about 3:1, and most preferably at least about 5:1.
It is also often desirable to include in a packaging film a shrink feature, i.e, the propensity of the film upon exposure to heat to shrink or, if restrained, create shrink tension within the packaging film. This property is imparted to the film by orientation of the film during its manufacture. Typically, the manufactured film is stretched in either a longitudinal (machine) direction, a transverse direction, or both, in varying degrees to impart a certain degree of shrinkability in the film upon subsequent heating. After being so stretched, the film is rapidly cooled to provide this latent shrinkability to the resulting film. One advantage of shrinkable film is the tight, smooth appearance of the wrapped product that results, providing an aesthetic package as well as protecting the packaged product from environmental abuse. Various food and non-food items may be and have been packaged in shrinkable films.
It is sometimes also desirable to orient a packaging film and thereafter heat set the film by bringing the film to a temperature near its orientation temperature. This produces a film with substantially less shrinkability, while retaining much of the advantages of orientation, including improved modulus and optical properties.
Of interest are U.S. Pat. Nos. 4,361,628, and 4,552,714, as well as 4,588,648 and 4,617,240 (Krueger et al) which disclose asymmetric films having polypropylene and nylon bonded by a polypropylene-based adhesive layer.
Also of interest is U.S. Pat. No. 4,726,984 (Shah) disclosing an oxygen barrier oriented film with a core layer of ethylene vinyl alcohol copolymer; two intermediate adhesive layers; and two outer layers of a blend of ethylene propylene copolymer and polypropylene.
It is an object of the present invention to provide a coextruded thermoplastic multilayer film characterized by a combination of good oxygen barrier and good abuse resistance properties
It is another object of the invention to provide a polymeric film characterized by a relatively low oxygen transmission rate, and a relatively high carbon dioxide transmission rate.
It is a further object of the present invention to provide a thermoplastic multilayer film having an aesthetic appearance with good clarity, and other desirable optical properties.
It is another object of the present invention to provide a thin thermoplastic multilayer film having superior toughness and abrasion resistance.
It is still another object of the present invention to provide a coextruded thermoplastic multilayer film which may be totally coextruded, and oriented to provide a film with good shrink properties and good oxygen barrier and abuse resistance properties over a wide range of moisture conditions.
It is yet another object of the present invention to provide a coextruded thermoplastic film which is oriented yet substantially shrink-free.